Security alarm systems commonly comprise appropriate sensor units associated with items to be protected against intrusion, which may either be some form of switch or circuit maker or breaker responding to interference by intruders with the item to be protected, or some form of transducer responding to deviation outside a predetermined limit of some condition associated with the item to be protected or its environment, e.g. light or movement detectors. In each case, an alarm condition will normally be signified by either the sending or the interruption of an electrical signal. The signals from the sensor unit or units are usually directed to a control unit in the vicinity of the item or items to be protected, and this control unit may contain an alarm responsive to an abnormal sensor unit signal, and/or a signal may be electrically transmitted to a remote point to be monitored for abnormalities. In the latter case, the security alarm signal may be encoded, for example by modulation with a pseudo-random digital sequence, to improve its security. Unfortunately encoding systems providing a high degree of security for electrical signals are relatively expensive and are usually economically impractical for application to the local connections between the sensors and the control units, which connections thus often form the weakest link in an otherwise secure system.